PROCEEDINGS OF THE 31st ICRC, ŁÓDŹ 2009

1Gamma ray point source search by GRAPES-3 experiment
A.Oshima∗ , S.R.Dugad∗, T.Fujii† , U.D.Goswami∗ , S.K.Gupta∗ , Y.Hayashi†, N.Ito† ,P.Jagadeesan∗, A.Jain∗ , S.Karthikeyan∗, S.Kawakami†, H.Kojima‡ , T.Matsuyama†,
M.Minamino† , H.Miyauchi†, P.K.Mohanty∗, S.D.Morris∗, P.K.Nayak∗, T.Nonaka†,
S.Ogio† , T.Okuda†, B.S.Rao∗, K.C.Ravindran∗, M.Sasano†, K.Sivaprasad†,
H.Tanaka†, S.C.Tonwar∗, E.Usui† , Y.Yamashita†∗ Tata Institute of Fundamental Research† Osaka City University, Japan‡ Aichi Institute of TechnologyAbstract. The GRAPES-3 experiment at Ooty inIndia, is an air shower array of about 300 plastic
scintillator detectors, and a large area (560 m2 )tracking muon detector with 1 GeV energy threshold.
The detectors are deployed on a hexagonal patternwith inter-detector distance of 8m. A good angularresolution is required for the search of gamma-ray
point sources which is also one of the main objectivesof the GRAPES-3 experiment. An angular resolutionof the array is ≤ 1◦ above a primary energy 20
TeV. The large area tracking muon detector is used
to measure the muon component of each detected
showers. The huge number of EAS due to charged
primary cosmic rays is the main background forthe gamma ray point sources search. Since the EAS
induced by cosmic rays contain larger number muons
due to their hadronic interactions, we may distinguish primary gamma-ray induced events from thehadron induced showers, by using the informationfrom the muon detectors. A gamma-ray signal fromthe direction of the CRAB nebula has been obtained
from the GRAPES-3 array which will be presented
and discussed.
Keywords: cosmic ray,gamma ray, Crab Nebula
I. I NTRODUCTION
The discovery of the pulsars in the 1960s further
added to the interest in particle acceleration processes
associated with supernovae, since the pulsars themselves
were shown to be capable of directly accelerating particles to TeV energies and beyond. Among the many
known pulsars and the associated supernova remnants,
the CRAB pulsar and its associated nebula have beenamong the most-studied astronomical objects at all
wavelengh in the electromagnetic spectrum.
Successful models for the production of γ-rays up toGeV energies, invoke the acceleration of the charged particles in the pulsar wind by the shock front to relativisticenergies, and the production of the γ-rays occurs throughthe synchrotron and curvature radiation processes. The
γ-ray energy spectrum of the CRAB nebula has beenmeasured up to TeV energies by the pioneering observations of the Whipple group [?]. Detailed studies on

the energy spectrum in the TeV energy region by therecent high resolution imaging Ĉerenkov telescopes suchas MAGIC, HESS and VERITAS etc., have enabledhighly successful modeling of the acceleration and γray generation processes occurring in the CRAB nebula.
These models visualize the production of the γ-rays,
through inverse Compton scattering of the synchrotron
photons by the synchrotron-emitting highly relativistic
electrons in the shock region. A fit to the TeV spectrum
requires the acceleration of particles up to energies∼ 1015 eV.
It is expected that a significant part of the energy
loss from the pulsar would be in the form of thehadronic component of the pulsar wind. The γ-rays
produced through the decay of neutral pions (π ◦ ) via
hadronic interactions, would have significantly different
energy spectrum than the photons produced by the
inverse Compton scattering from the electrons. Since
the lifetime of TeV electrons in the nebular magnetic
field decreases very rapidly with increasing energy, asteepening of the energy spectrum is expected in the
10-100 TeV region. On the other hand, the spectrumof the γ-rays produced via π ◦ decays may extend theenergy spectrum beyond 10 TeV, without much change
in the slope. Therefore, there is considerable interest,
in exploring the shape of the energy spectrum in the10-100 TeV region, in order to determine the relative
contributions of these two competing processes for theproduction of the γ-rays in the CRAB nebula.
We present here, the results from a 5-year study of theshowers arriving from the direction of the CRAB nebula,using the muon-poor criterion for their selection as γ-ray
candidate events.
II. GRAPES-3 EXPERIMENTThe experimental system of the GRAPES-3 (Gamma
Ray Astronomy at PeV EnergyS Phase-3) experimentconsists of a densely packed array of scintillator detectors and a large area tracking muon detector. The
EAS array consists of 257 plastic scintillator detectors
shown in Fig. 1, each of 1 m2 in area. These detectors
are deployed with an inter-detector separation of only
8 m. The array is being operated at Ooty in south India(11.4◦N, 76.7◦ E, 2200 m altitude).

2A.OSHIMA et al. GRAPES-3 POINT SOURCE SEARCH

In order to achieve the lowest possible energy threshold, a simple 3-line coincidence of detectors has beenused to generate the Level-0 trigger, which acts as thefast GATE and START for the analog to digital and time
to digital converters (ADCs and TDCs), respectively.
As expected, this trigger selects a large number ofvery small and local showers and also larger showers
whose cores land very far from the physical area of thearray. Therefore, it is also required that at least 10 outof the inner 127 detectors should have triggered theirdiscriminators within 1 µs of the Level-0 trigger. This
Level-1 trigger with an observed EAS rate of 13 Hz isused to record the charge (ADC) and the arrival time(TDC) of the pulses from each detector [4]. The pulse
charge is later converted into the equivalent numberof minimum-ionizing particles (MIPS) using the mostprobable charge for a single MIP measured using thetrigger from a small area (20×20 cm2 ) scintillation
counter telescope.
The 560 m2 GRAPES-3 muon detector [9] consists of 4
super-modules in Fig. 2 , each in turn having 4 modules.
Each module with a sensitive area of 35 m2 consists ofa total of 232 proportional counters (PRCs) arranged
in 4 layers, with alternate layers placed in orthogonal
directions. Two successive layers of PRCs are separated
by 15 cm thick concrete. The energy threshold of 1 GeV
for vertical muons, has been achieved by placing a totalof 15 layers of concrete blocks (total absorber thickness∼550 g.cm−2) above the Layer-1. The concrete blocks
have been arranged in the shape of an inverted pyramid
to provide adequate shielding up to a zenith angle of45◦ .
One of the most critical parameter in the search for point
sources of cosmic γ-rays, using a particle detector arrayis good angular resolution. This requires an accuratedetermination of the relative arrival time of the shower
front at various detectors. The high density of thedetectors in GRAPES-3 enabled an angular resolutionof 0.7◦ to be obtained at energies as low as 30 TeV.
Angular resolution of the GRAPES-3 was estimated by
2-D Gaussian fit to the Moon shadow data.
III. DATA AND A NALYSIS
A total of 1.9×109 showers have been collected overa total live time of 11.9×107 s, spread over a 5-year
period, from 2000 to 2004. For each EAS, the core
location, the shower age ‘s’ representing the steepnessof the Nishimura-Kamata-Greisen (NKG) lateral distribution function and the shower size Ne have beendetermined using the observed particle densities, following the minimization procedure discussed in detail by
Tanaka et al [6]. Also, for each shower, the zenith (θ)
and the azimuth (φ) angles have been calculated using
the time information from the TDCs, also following theminimization procedure described by Tanaka et al [6].
It is very difficult to observe the tiny flux of γ-rays fromvarious interesting astrophysical sources in the sea of nuclear cosmic rays whose direction has been randomized

Fig. 1. The GRAPES-3 experimental system with 257 scintillator
detectors and 16 muon detector modules

Fig. 2.
A muon station has four muon detector modules eachconsisting of 232 proportional counters. There are four muon stations
inside the air shower array (Fig. 1).

3.210

3.5

3.5

<= Ne < 1010100<= Ne < 103.75

3.75

10100

100

10104.0<= Ne < 101010.1

101 4.0 23 4.25
410<= Ne < 105100
1010.1

101 4.25 2
344.510<= Ne < 1051001010110.1

0.10.0101 4.75 2
345.0
10<= Ne < 105150100

1 4.5 2344.75
10 <= Ne < 105150.01
0123450234100
1010.1
0.01
0234Fig. 3.
Distribution of the number of muons accompanying theobserved air shower for each shower size range.

by the interstellar magnetic field. At higher energies,
where observations have to be necessarily made withparticle detector arrays at high altitudes, the muon content of showers offers itself as a possible parameter todiscriminate against showers initiated by nuclear cosmicrays. Fig. 3 shows the GRAPES-3 observations on themuon content of showers.
Fig. 4 shows the distribution of the number of showers
in various annular regions centered on the direction of
the Crab nebula for the complete data set of nearly 5years, for N e ≥ 104.0 which satisfied the ’zero-muon’
and ’muon-poor’ criteria. It is interesting to note a small

PROCEEDINGS OF THE 31st ICRC, ŁÓDŹ 200933×107000

340

6000
5000

320

[sec]

4000
3000
2000

300

1000

2800000.5

1

1.5

2

2.5

33.5

Fig. 4. Distribution of the number of showers in various annularregions centered on the direction of the Crab nebula for a sub-setof the complete set of showers , which satisfy the ’zero-muon’ and’muon-poor’ criteria.

excess (2.1σ) flux from within 2◦ of the Crab nebulafrom a Gaussian fit to the data plotted in Fig. 4.
It is evident that the statistical significance of the excess from the direction of the Crab nebula for the ’zeromuon’ and ’muon-poor’ showers is not large and not
beyond the possibility of being a statistical fluctuation.
However, it is still of considerable interest to compare
the observed flux with other observations at TeV-PeV
energies, specially in view of the fact that the present
results possibly represent the first positive detection of
the gamma ray flux from the Crab nebula using the’zero-muon’ and ’muon-poor’ criteria with one of thelargest muon detectors used for this purpose. It may
be noted here that the background flux, required for
estimating the excess from the direction of the Crabnebula is rather well-determined and agrees well among
the three observation regions.
In general, the integrated intensity of gamma raysfrom an astronomical object can be expressed as follows:
Z EcdN
I=dE
(1)
Et dE

where Et is the effective threshold energy for detection of gamma rays with the GRAPES-3 array and Ec
is the assumed cut-off energy for the energy spectrum.
Ec is being taken to be 1014 eV here. The differential
intensity can be assumed to be given by the relation: −αEdN
dE = I0(2)dE
E0
where α is the power-law exponent for the spectrum
in the 10-100 TeV energy range and I0 is the differential
intensity at a value E0 for the γ-ray energy. Combining
the above two equations, the integrated intensity I isobtained as follows,I==Z E2

−αEdE
I0E0E1"  −α #−αEc
I0 E0
E1−−α + 1
E0E0102030Zenith angle [deg]440Fig. 5. Exposure time for the Crab nebula as a function of the zenith
angle in 5◦ intervals.

The observed flux from the Crab nebula, integrated
over the time interval, between times t1 and t2, may be
written as,
I=Z Ec Z t2
Ett1

dN
Aef f (E, θ(t))dtdEdE(5)where Aef f (E,θ(t)) is the effective area of the shower
array for the γ-ray energy E and θ(t) is the zenith angleof the Crab Nebula at time t.
Fig. 5 shows the variation of the exposure time for theCrab nebula with the zenith angle θ(t) in 5◦ intervals.
we have converted the shower size Ne into the energy ofthe primary protons, using the CORSIKA Monte Carlosimulation program, version 6.500 in Fig. 6 [8]Using the values of the exposure times for various zenith
angles shown in Fig. 5 and converting the observed
shower size to primary energy of gamma rays using
the energy-size relation shown in Fig. 6, the integral
flux values have been determined for 5 values of theprimary energy. For example, the integral flux above anenergy threshold of of 9 TeV is found to be (5.08 ± 2.67)·10−13 cm−2 s−1 and (6.92 ± 2.87) ·10−14 cm−2 s−1 forenergy above 30 TeV. These results are plotted in Fig.
7 for obtaining the gamma ray energy spectrum for theCrab nebula. The power-law index for the energy spectrum determined from these observations is 1.70±0.06
over the energy range, 9–70 TeV. In the same figure arealso shown various other measurements, most of themobtained from observations with imaging atmospheric
Cherenkov telescopes, except for the result from Tibet
which was also based on observations with particle
detector array.
It is interesting to see from Fig. 7 that a single powerlaw seems to fit all the measurements from ∼500 GeV toenergies ∼100 TeV and there is no evidence to suggest
any steepening of the spectrum, at least upto energy∼100 TeV.

(3)

IV. D ISCUSSION(4)

The results presented in Fig. 7 also show that themeasurements right up to energy ∼ 100 TeV remain
consistent with the predictions of the ’inverse-Compton

4

A.OSHIMA et al. GRAPES-3 POINT SOURCE SEARCH
TeV
500
Integral Flux (cm-2s-1)

Crab Nebula Integral Flux

10020104105

NeFig. 6. The x-axis shows the shower size Ne and corresponding
primary proton energy in TeV, is shown on the y-axis.

scattering’ model which requires only the accelerationof high energy electrons to PeV energies in the CRAB
nebula. There is still no pressing need to invoke thepresence of PeV energy hadronic cosmic rays in theCRAB nebula. However, any observation extending thespectrum to energy ∼ 1000 TeV without significant
steepening is likely to make a strong case for a large
contribution of the hadronic cosmic rays to the observed
γ-ray flux. In this context, it is relevant to mention herethat the GRAPES-3 array is expanded to increase thecollection area and the size of the muon detector isalso being increased. These additions to the GRAPES-3
system should help in extending the observations on thegamma ray energy spectrum for the CRAB nebula toenergies ∼ 1000 TeV.
V. S UMMARY
We have analyzed air shower data collected by theGRAPES-3 experiment over a 5-year period from 2000
to 2004. A search for an excess flux due to γ-rays fromthe CRAB nebula over the complete data set has yielded
negative result. However, using the discriminating powerof the muon component of air showers to improve thesignal to background, we have observed an excess flux
in showers selected with ’zero-muon’ and ’muon-poor’
criteria. These observations yield an integral flux of (5.08± 2.67) ·10−13 cm−2 s−1 above an energy threshold of
9 TeV and (6.92 ± 2.87) ·10−14 cm−2 s−1 for energy
above 30 TeV. The observed flux is in good agreementwith the large-angle observations by the CANGAROO
collaboration with atmospheric Ĉerenkov telescope andalso with the results reported by the Tibet AS/γ collaboration. The energy spectrum shows a single power-law fit
over the broad energy range, 500 GeV to 100 TeV, consistent with the expectations from the ’Inverse-Compton
scattering’ model for the acceleration of electrons andproduction of γ-rays in the CRAB nebula. An on-going
expansion of the GRAPES-3 particle array as well as theenlargement of the muon detector is expected to help inthe extension of observations on the energy spectrum toenergies ∼ 1000 TeV which would permit a distinction-910HESS
HEGRA

1-10

WHIPPLECANGAROO

Tibet

10-11

GRAPES-3

10-12-131010-14
10-15
-16

10

10-1

110102
Energy (TeV)

Fig. 7. Integral energy spectrum of gamma rays from the Crab nebula.
The flux values obtained from the present experiment are in good
agreement with the results given by the CANGAROO and Tibet AS/γ
collaborations. However, the flux observed by the HESS collaborationis almost 50% larger than the values obtained from the GRAPES-3
experiment.

between electron and hadronic processes for the γ-ray
production in the CRAB nebula.
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